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Operating A Hydronic Boiler for Maximum Efficiency

December 3, 2013

Many facilities are replacing their steam boiler with a hydronic one to increase efficiency and decrease system operating cost. Before condensing boiler technology was introduced, boilers operated at a maximum of 80% to 85% efficiency. Today, condensing hydronic boilers can operate at efficiencies in the mid to high 90s. Hydronic boilers can be used for either building heat or process hot water applications. 

Some facilities that convert from a steam boiler to a condensing one reduce their energy bill as much as 50%. A condensing boiler is more efficient than a steam boiler because it extracts both latent heat and sensible heat from combustion exhaust. 

Steam systems are prone to heat loss for a few reasons. First, many systems utilize a steam-to-water heat exchanger, while others have direct steam heating equipment. Energy is lost in these systems through steam traps, steam leaks as well as radiant losses through the piping because steam systems run at a high temperature. Some facilities add more insulation around the piping, which minimizes the heat loss, but it does not eliminate it.

To achieve a substantial energy savings with a hydronic system, a facility must take three steps: 

  • Initiate an outdoor reset schedule
  • Implement a night/weekend setback scheme 
  • Operate the system at a larger system temperature differential with lower return water temperatures

With outdoor reset, a facility manager enters the outside air temperature into the control or building management system, and the system adjusts to meet the need.  When it's 0 degrees outside, 180 degree supply water typically is required to maintain building comfort; however, when it's 60 degrees outside, only 120 degree supply water may be needed to heat the building.  So, the supply water temperature can be scaled back while maintaining the same comfort level, thereby saving energy.

The night/weekend setback scheme is easy to understand. If there are only a few people in the facility overnight or on weekends, a facility manager decreases the header temperature 10 to 20 degrees less than what it would normally be.  The facility manager sets the temperature to warm-up the building an hour before people arrive.

While the night/weekend setback scheme is logical, setting a larger system temperature differential is not for most engineers.  A 20-degree differential is commonly accepted in the industry as the norm. The heating coils and air handler are sized to meet a supply temperature of 180 degrees out, returning at 160 degrees. However, if the supply temperature stays at 180 degrees, but the returning temperature is decreased to 140 degrees, the building temperature stays the same, but the system only requires half of the water flow because heat load is directly proportional to the differential temperature and flow rate.  If the heat load is the same, and the differential temperature is doubled, the flow rate is cut in half. 

There are a couple of benefits to this system.  First, with less flow, a smaller pump or variable speed pump can be used, which increases electrical energy savings.  Also, if the supply temperature goes out at 180 degrees and comes back at 160 degrees, there is no condensing and peak efficiency is limited to below 90%. But, if the system supply goes out at 160 degrees on a reset schedule and comes back at 120 degrees, the system will condense sooner with the larger Delta-T.  If the supply temperature goes out at 120 degrees and comes back at 80 degrees, the system is capable of condensing all of the time.  A larger differential temperature with a condensing boiler can drive the system into condensing mode sooner, thereby reducing fuel consumption in addition to saving electrical energy in the system due to the smaller pump or reduced pump speed. 

It is important to note that under similar conditions, a non-condensing boiler cannot get much below 160 degrees with a 140 degree return, because below 140 degrees, condensing will begin in the non-condensing boiler and eventually destroy it. Traditional hydronic systems were originally designed to protect the equipment, not provide maximum system efficiency.

Condensing Curve

In existing buildings, changing the temperature differential can be hard to do, but in new buildings it is easy, particularly with certain hydronic boiler controls. With select controls, pumps can be set to maintain a constant differential temperature. For example, if the system is set for a 40 degree differential temperature, and it drops to 36 degrees because the heat load decreases, not as much heat is extracted from the water, so the pump slows down. The water volume decreases when the pump slows down, which draws out more heat, returning to the desired 40 degree Delta-T.  In addition, slowing down the pump speed increases boiler efficiency, and thus reduces energy usage. 

It is important to note that if a facility installs condensing boilers and operates them at a supply water temperature of 180 degrees out and 160 degrees back all the time, the boiler never kicks in to condensing mode and the benefits of a condensing boiler are never fully realized. In fact, systems can become less efficient since many condensing boilers have a small water volume and will tend to have more cycling and purge losses. This scenario is all too common. Implementing the boiler control and piping system correctly is critical to achieving the maximum efficiency benefit with condensing boilers. 

It is often advantageous to use multiple boilers in condensing applications. Multiple boiler systems are designed to provide proper system turndown to meet the peak load and minimum design load conditions, as well as provide sufficient redundancy. By installing multiple condensing boilers, a facility manager can stage the boilers depending upon heating load, which helps save fuel compared to one larger boiler. 

Along with multiple boilers comes the challenge of controlling them to operate at their peak efficiency. In contrast to traditional steam and non-condensing hydronic boilers, condensing boilers have an inverse efficiency curve characteristic. This means that condensing boilers operate most efficiently at lower firing rates. A good control strategy will modulate multiple condensing boilers at lower firing rates to keep the system operating at peak efficiency.

Are All Condensing Boilers the Same?

There are many different designs of condensing hydronic boilers, and some better than others.  Published efficiency ratings do not tell the whole story. Heat exchanger design, materials of construction, and effective heating surface area are important factors that should be evaluated when selecting a condensing boiler.

Many boilers are designed to meet dimensional and material constraints first, with operational performance and efficiency a distant second. While a small footprint and low cost are desirable and important considerations, the operational efficiency and reliability are often compromised. In evaluating the design and selection of equipment for a new or upgraded hydronic system, the lifetime operating cost, or total cost of ownership, should be considered.

The most effective heat exchangers are counter-flow, or counter-current, arrangements and deliver the maximum amount of condensing possible. Cold return water is introduced at the end of the heat exchanger with the coldest exhaust gases while the hot supply water is alongside the hottest combustion gases. Some boilers will only condense at reduced firing rates due to limited effective heating surface (per Btu input) and compromised heat exchanger design. 

Certain boiler designs will also limit the system piping options, and correspondingly, the potential efficiency gains. Non-condensing designs, such as copper-finned water tube or cast- iron sectional boilers, may be packaged with a secondary heat exchanger, typically made of stainless steel, to obtain condensing performance. These boilers cost less to manufacture, and have very little water volume and must be piped with a dedicated circulating pump and temperature control to ensure adequate water flow and temperature to protect the heat exchanger. Operational efficiency and long-term reliability are compromised with these designs.

Condensing stainless steel firetube boilers deliver more effective heating surface in a larger water volume design. Besides delivering higher operational efficiencies, these provide the customer and engineer with more flexibility in system piping and variable flow opportunities. Large water volume firetubes can be implemented in either primary pumping or primary-secondary arrangements. The firetube heat exchanger is conducive to counter-flow arrangement and maximizing the effective flue-side heating surface available. The large water volume inherent in firetube designs deliver low-flow tolerance making them ideal for variable flow primary pumping system. 

A Hybrid System Offers Flexibility

Facilities such as hospitals, universities and large commercial buildings are utilizing a hybrid system comprised of condensing and non-condensing boilers. The greatest benefit of a hybrid system is its flexibility. With an intelligent hybrid system control the appropriate boilers come online as needed and are controlled to operate at optimum performance points, thereby maximizing efficiency and significantly reducing first cost, operating costs, as well as providing the best total lifecycle cost of ownership.

Hybrid systems are especially advantageous for facilities that operate in colder climates. From mid-December to mid-February, when the temperature is 0 degrees or below, it may be best for a building to run a non-condensing boiler. Given a 180 degree supply and 160 degree return water temperatures in these conditions, there is little difference in the efficiency of a condensing boiler versus a non-condensing boiler.

Minimal Maintenance

Maintaining a hydronic boiler system is often easier than maintaining a steam one.  A boiler operator must closely manage the chemical treatment in a steam boiler. As the system evaporates water into steam, the chemicals and minerals stay inside the boiler and can become highly concentrated.  As a result, the pH level can spike. Daily boiler blowdown and water sampling are necessary for maintaining safe and reliable steam boiler operation.

A hydronic boiler is incorporated in a closed-loop system, so the chemicals that are added are significantly reduced. Unlike in a steam system, there is very little make-up water required. In a closed loop hydronic system, there is less opportunity for undesirable constituents to enter the system, such as hardness, oxygen and carbon dioxide. Typically operators only have to evaluate the chemistry in a closed-loop system once a month and make any necessary adjustments.  A water meter on the make-up line is recommended to help determine if a leak is present and if adjustments to the chemicals are required.

Maximizing Your Return on Investment

The payback on a new hydronic system is typically two to four years; however it can be less depending on how inefficient the existing boiler operation is. The payback on a hybrid system is often shorter in the case where a facility buys condensing boilers and a hybrid control system to supplement its existing non-condensing boiler. The energy savings for this type of retrofit typically is between 25% and 30%.  Increases in efficiency directly correlate to effective control management that maximizes the run time of the condensing boilers while delivering the heat required to meet load demands.  

To learn more about hydronic heating systems and how they can help your company reduce energy costs, contact your local Cleaver-Brooks representative or visit cleaverbrooks.com.

Article written by Sean Lobdell and featured in the November/December 2013 issue of Commerical Building Products 

Download: Operate An Efficient Hydronic Boiler